The invention relates to piezoelectric switches.
It is well known in the art that an oriented polyr-methyl glutamate film or a polar thermoplastic film such as a polyvinylidene fluoride or polyvinyl fluoride film which is polarized under a high d.c. voltage electric field, has a high piezoelectric property. Piezoelectric switches, which comprise a piezoelectric thermoplastic film element with electrodes attached to both surfaces thereof, are subjected to a stress to produce a piezoelectric field for a switching operation. Such switches are disclosed in Japanese Opened Patent Application No. 47-37244.
In such conventional piezoelectric switches, the electric field is temporarily obtained only at the moment when the stress is applied to or removed from the piezoelectric film element, and is not held permanently. Further, the resulting electric field is indicated as a differential value according to changes in stress. Accordingly, the conventional piezoelectric switches cannot be used as switches for remove sensors which require an absolute value.
U.S. Pat. No. 3,935,485 to Yoshida et al describes a piezoelectric switch utilizing a thermoplastic film element. The switch shown is an example of the conventional piezoelectric switches mentioned above. Only a momentary output signal can be obtained by a single push operation since the film element is defined by a push button.
It is also known in the prior art to provide a piezoelectric switching system wherein a mechanical oscillator is brought into contact with an inorganic piezoelectric element for providing a continuous electrical output. U.S. Pat. No. 3,749,947 to Kawata et al describes an example of such a piezoelectric switching system. In this case, when a push button is actuated to bring the mechanical oscillating element into contact with the inorganic piezoelectric element, a continuous electric signal is provided between the electrodes. In FIGS. 2 and 4 of the Kawata et al patent there is shown a piezoelectric switching system wherein a driving element 13 which is a mechanical piezoelectric transducer is attached to a resonating member 14. The intermediate points of the resonating member are brought into contact with the inorganic piezoelectric members 4A through 4N by means of a push button to individually actuate the latter piezoelectric transducers.